The present invention relates to a method and apparatus for measuring a water content by using a thermal conductivity and, more particularly, to a method and apparatus for obtaining a water content in a soil from the thermal conductivity of the soil.
Conventionally, when an amount of water (water content) of soil is to be measured, sampling is performed every time measurement is required, and the water content of the sample is measured in a laboratory. If the water content must be managed, for example, the valve of a sprinkler is controlled by the operator in accordance with the water content obtained by the measurement. In management of the water content in banking, an RI (Radioactive Isotope) is detected from the surface of the ground. The count of the RI counter that changes in accordance with the water content of the soil is used to measure the water content in the banking.
However, when a water content is to be measured by sampling, sampling must be performed every time the water content is to be measured. This requires a cumbersome and time-consuming operation. When a water content is to be measured using an RI counter, the RI counter is as very expensive as several million yen, and water content of only the surface layer of the earth of about several tens cm of depth can be measured.
A theory of heat flow conduction is developed from a Fourier's basic heat conduction equation. In this equation, the nature of heat conduction is included in the thermal conductivity for the sake of simplicity. This theory is correct when a correct thermal conductivity is used. Therefore, efforts have been concentrated on measurement and study of thermal conductivity.
Conventional methods for measuring a thermal conductivity are classified into steady-state and unsteady-state methods. According to the steady-state method, high precision is obtained when measurement is performed at room temperature. However, it is difficult to maintain a steady state at high temperatures, and it requires a long period of time for measurement. In contrast to this, according to the unsteady method, measurement can be performed within a shorter period of time, the size of a sample can be small, and measurement can be performed at both high and low temperatures. Therefore, recently, when thermal characteristics of a solid, such as ground, rock, and concrete, are to be measured, the unsteady-state method is adopted.
Examples of the unsteady-state method of this type include a thermal conductivity measuring method called a thermal probe method. According to the thermal probe method, a needle-like thermal probe having a pair of a temperature sensor and a heater is inserted into a sample. The heater in the probe is caused to generate heat and the temperature change and time are measured. The thermal conductivity is obtained based on the temperature change.
When the heater in the thermal probe is heated to increase the temperature, heat conduction occurs in the sample. In this case, when the thermal conductivity of the sample is high, the heat generated by the heater is easily dissipated. However, when the thermal conductivity of the sample is low, the heat generated by the heater cannot be easily dissipated. As a result, the temperature of the heater is abruptly increased with a steep ramp. The time and the temperature change in this case exhibit a certain correlation with each other when the time base is expressed as a logarithm. As a result, the thermal conductivity can be calculated from the gradient of the linear portion of the graph.
However, the conventional method described above has the following problems.
When calculation of a thermal conductivity is to be performed, the sample must be classified in order to calculate a heat capacity to be applied. The current must be determined and the obtained predetermined current must be flowed to the heater to heat it. With the conventional method, required voltage and current cannot be obtained immediately after the start of power supply because of the transition phenomenon or the like of the power supply. As a result, the reliability of the initial data cannot be improved. More specifically, since the thermal conductivity also depends on temperatures, when the heater is heated after power supply is started and before required current and voltage are obtained, the temperature of the test piece is increased, and the precision of the test result immediately after the start of power supply is degraded.